In this narration the Holy Prophet declared most crual person is the one among people who killed the Camel of Hazrat Salih (as )and most crual person after him is the person who will kill Ali (as).
Ammar bin Yassar narrated:
Ali ibn Abi Talib and myself participated in one war for Islam. The Prophet stayed on one place, we say the people of Banu Madallaj they were working in their garden. Ali ibn Abi Talib said to me “Oh Abu Yaqzan, Do you want us to see how this people work in their fields.” I said “If you want we can go and see them.” We went there and we saw them for a while. Then we felt sleepy, we left that place and we came on another place and we lay down on the ground under the shadow of the Date tree. We slept there. Then I swear to Allah that no one waked us except the Holy Prophet. The Holy Prophet was moving us with his feet, we got dust on our body because of lying down on the ground. That day the Prophet said to Ali “Oh Abu Turab, Do I not tell you the most cruel person among the people?” We asked the prophet “Oh Prophet of Allah, please tell us.” The Prophet said “The person who killed the red female camel of the nation of Thamud and that person who will hit you on this place (and prophet put his hand on that place and hold the beard of Ali..”
Part B urdu translation